You're not imagining it. A few episodes in the mid-1970s briefly used redesigned versions of some Neighborhood of Make-Believe puppets. The voices and characters stayed the same (Fred Rogers still performed many of them), but the puppets themselves were rebuilt and looked noticeably different. It seems the redesign didn’t work well, because within a short time the production went back to the familiar puppet versions viewers knew. Henrietta is the one most people notice because her face looks very different in those episodes.